In recent years, polymer electrolyte fuel cells (PEFC) have been drawn attention as a power source for electric vehicles. Polymer electrolyte fuel cells have been adapted in practical use for various applications as they generate electricity at ordinary temperature.
Generally, a fuel cell system is divided into the cathode and the anode by a solid polymer electrolyte membrane. The cathode and the anode are placed in opposite relation across the solid polymer electrolyte membrane. The fuel cell system is supplied with oxygen in the air that is fed to the cathode and hydrogen that is fed to the anode, so that oxygen and hydrogen are chemically reacted to generate electricity for driving an outer load.
The following methods are generally known to supply a fuel for the fuel cell as a hydrogen supply source.
(1) Pure hydrogen-fueled type wherein a liquid hydrogen storage tank, a high pressure tank, or a hydrogen storage material such as hydrogen absorbing alloys is supplied with hydrogen in the form of pure hydrogen such as liquid hydrogen or high-pressured hydrogen to provide a hydrogen supply source.
(2) Reforming type wherein hydrogen is produced by means of steam reforming by the use of hydrocarbon such as methanol solution.
Meanwhile, as hydrogen supplying methods at a high pressure hydrogen supplying-type hydrogen station where high pressure hydrogen is supplied to a hydrogen storage tank mounted on the vehicle, the flowing methods are known.
(1) Hydrogen that is produced at a combinat followed by liquefaction is transported to a hydrogen station, and the liquid hydrogen is vaporized and pressurized at the hydrogen station.
(2) Organic gas such as natural gas, methanol, or organic liquid fuel such as gasoline is reformed on site by the use of a reformer, and the reformed gas is pressurized and supplied.
(3) Hydrogen is obtained on site from organics, metal complex/chemical hydrides, etc., and the obtained hydrogen is pressurized and supplied.
However, in these hydrogen supplying methods for supplying high pressure hydrogen, impurities are often mixed into hydrogen by the following reasons.
(1) When organic gas such as natural gas, methanol, or organic liquid fuel such as gasoline is reformed on site by the use of a reformer, carbon monoxide gas is generated during the fuel reforming process and the generated gas is mixed into hydrogen as impurities.
(2) Contents of lubricating oil that is used for lubrication of a hydrogen pressurizing apparatus during the pressurizing process of hydrogen are mixed into hydrogen as impurities.
(3) Extremely small amount of moisture or contaminants are mixed from a hydrogen supply hose as impurities.
(4) If a reforming catalyst of the reformer that is used for the hydrogen production process is not in good condition, by-products other than hydrogen are produced and mixed into hydrogen as impurities.
(5) Because of accumulation of these impurities, purity (concentration) of supplied hydrogen may become lower than the allowable hydrogen purity (allowable hydrogen concentration) of the fuel cell.
Performance of the fuel cell decreases seriously if low purity hydrogen including a great amount of impurities is used.
For example, if impurities mixed into a hydrogen fuel in the fuel cell restrict a chemical reaction at the electric pole, generation of power voltage decreases at the fuel cell, that is, the output of the fuel cell decreases. For this reason, in an on-board fuel cell provided with a hydrogen storage/supplying apparatus, it is necessary to prevent impurities from being supplied to the fuel cell by taking measures at utility device (on-board) side with respect to the impurities mixed into hydrogen that is supplied from the hydrogen station (on-site).
As shown in FIG. 8, a membrane separation device 102 is known as a hydrogen separator where only hydrogen is separated from low purity hydrogen including a great amount of impurities. When hydrogen including impurities (i.e., carbon monoxide gas) is supplied to the membrane separation device 102, hydrogen passes through the separation membrane whereas impurities do not pass through the separation membrane, so that the amount of impurities that passes through the membrane separation device 102 decreases and the purity of hydrogen is improved accordingly. Meanwhile, the concentration of impurities (carbon monoxide gas) that have not allowed to pass through the separation membrane increases in the membrane separation device 102. If pressure at the low purity hydrogen side (upstream of the separation membrane) is constant, purifying rate at the separation membrane decreases due to increased partial pressure of the constituents of impurities and decreased partial pressure of hydrogen. For this reason, it is necessary to remove or release the impurities-containing gas (bleed gas). In this instance, hydrogen at the low purity hydrogen side is discharged together.
In view of the aforementioned drawbacks, the object of the present invention is thus to provide a hydrogen supplying apparatus for a fuel cell, which enables hydrogen supplied to the fuel cell to be retained not less than a predetermined purity (concentration) and further improves the utilization efficiency of hydrogen supplied to the fuel cell.